I think considering the definiton of the unexchanged balance may also clarify some of this. The unexchanged balance is an estimate of the value of a portfolio of altcoins you've previously mined which have not yet been exchanged.
Now say I'm Miner 2 (I'm not) and I mined on the US server all day Friday and we were mining a coin that was not autotrading (so my unexchanged was shooting up). Now imagine we changed the coin we were mining at some point on Saturday and you started mining on the US server at some point after. So now my portfolio has all these coins I mined on Friday that have not been exchanged yet, and your portfolio has none of these coins because you were mining on the EU server on Friday (for example). Now we mine all day and our balances grow at similar rates (as your graphs suggest they do, the slopes being fairly similar all things considered), but at the close of Saturday our portfolios are drastically different because I still have all these coins I mined on Friday that you don't have. Now if H2O decides its time to sell those coins (the ones mined on Friday), well my unexchanged plummets and my balance shoots up, but yours does not because he didn't sell the coins that were in your unexchanged portfolio.
To address your point about how Miners 2,5, and 6 returned to essentially the start of day unexchanged values, well thats likely a product of the how much unexchanged they accrued over the day that wasn't whatever coin was mined on Friday, and hence these coins acted exactly the same as yours, give or take some variability for differences in the altcoin composition of everyone's portfolio based on which days you've mined what servers in the past.
Now I don't know if this is how H2O approaches everyone's share of the unexchanged balance, and this assumes no daytrading of altcoins that squeeze out every last mBTC, but it seems like a potential explanation.